Preparation of Ce-MnOx Composite Oxides via Coprecipitation and Their Catalytic Performance for CO Oxidation

Ce-MnOx composite oxide catalysts with different proportions were prepared using the coprecipitation method, and the CO-removal ability of the catalysts with the tested temperature range of 60–140 °C was investigated systematically. The effect of Ce and Mn ratios on the catalytic oxidation performance of CO was investigated using X-ray diffraction (XRD), X-ray energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM), H2 temperature programmed reduction (H2-TPR), CO-temperature programmed desorption (CO-TPD), and in situ infrared spectra. The experimental results reveal that under the same test conditions, the CO conversion rate of pure Mn3O4 reaches 95.4% at 170 °C. Additionally, at 140 °C, the Ce-MnOx series composite oxide catalyst converts CO at a rate of over 96%, outperforming single-phase Mn3O4 in terms of catalytic performance. With the decrement in Ce content, the performance of Ce-MnOx series composite oxide catalysts first increase and then decrease. The Ce MnOx catalyst behaves best when Ce:Mn = 1:1, with a CO conversion rate of 99.96% at 140 °C and 91.98% at 100 °C.


Introduction
A fresh focus on CO emissions has evolved as the issue of air pollution garners increasing attention in many countries, and strict emission standards have also been proposed in plans. In East Asia, the main source of CO is industrial sources. However, the CO concentration in East Asia decreased with an annual trend of 0.41 ± 0.09% between 2005 and 2016, with China alone accounting for 84% of the CO reduction [1]. In Pakistan, carbon monoxide mainly comes from industrial production and transportation, with car emissions, fossil fuels, and biomass combustion being the main sources of CO, and pollutant levels in congested areas are higher than in open areas [2]. Catalytic oxidation [3], in which CO is oxidized to CO 2 by a catalyst under certain conditions, is one of the most effective ways to remove CO in the face of these extremely stringent criteria and considerable emissions.
At the moment, the catalyst system used for CO oxidation can be categorized as noble metal catalysts (such as Pt, Au, and Pd) [4][5][6], as well as non-noble metal catalysts represented by metal oxides and composite metal oxide catalysts [7][8][9]. Non-noble metal catalysts have drawn a lot of interest in comparison to noble metal catalysts because of their low cost and abundant raw material sources. Manganese is frequently utilized in Inc., Shanghai, China. N 2 (99.9% concentration), CO (2% concentration), and O 2 (99.9% concentration) were purchased from Changsha Fanggang Gas Co., Ltd. Changsha, China.

Preparation of CO Oxidation Catalyst
The Ce-MnO x composite oxide catalyst was synthesized via coprecipitation method. A certain amount of cerium nitrate hexahydrate and manganese acetate tetrahydrate were dissolved in deionized water using a mass ratio of Ce:Mn = X:1 (where X = 3, 2, 1, 1/2, 1/3). After 1 h of magnetic stirring, ammonia was slowly added drop by drop at a constant rate of 10 s/drop until the solution pH reached 10. After 2 h of continuous stirring, the solution was allowed to stand for another 10 h until sediment occurred. The sediment was subjected to several washes with suction filtration and deionized water until the pH reached 7, and the precursor was obtained. The precursor was then dried in a blast oven for 5 h at 120 • C and calcined for 4 h at 400 • C in a muffle furnace with a heating rate of 5 • C/min. After that, the prepared catalyst was then cooled naturally to room temperature. Finally, the sample was sieved with 40-60 mesh, and the screened particles were selected for catalyst activity test.

Catalyst Activity Test
In order to test the catalyst activity, a micro-fixed bed reactor was utilized. Ce-MnO x composite oxide catalyst particles were measured using a 5 mL cylinder, and a gas space velocity of 30,000 h −1 was set. N 2 serves as the equilibrium gas in the mixed gas, which also contains an O 2 concentration of 16% and CO concentration of 6000 ppm. The DX4000 flue gas analyzer manufactured by Gasmet Technologies Oy, Vantaa, Finland was utilized to analyze CO concentration before and after the reaction in real time. The performance of catalyst was examined between 60 • C and 140 • C, with the test temperature being adjusted step by step. The CO conversion rate of the catalyst was calculated by the following formula:

Characterization of Catalysts
The phase analysis was detected by the D8 ADVANCE, Bruker, Karlsruhe, German using a Cu-K α source with a scan rate of 5 • /min and a step size of 0.02 • and a 2θ angle ranging from 5 to 80 • . The structure and element distribution of the sample were examined using Talos F200i, Thermo Fisher Scientific Inc, Massachusetts, America. The specific surface area was determined using Quadasorb SI-3, Anton Paar QuantaTec Inc., Boynton Beach, FL, USA. The surface microstructure was analyzed with JSM-6490LV, JEOL, Tokyo, Japan. Catalytic reduction capacity and surface CO adsorption-desorption were assessed using the Autochem II 2920, Micromeritics Instruments Corporation, Norcross, GA, USA. Detailed CO-TPD procedures were as follows: weigh and load 0.1 g sample into a U-shaped tube filled with quartz cotton. Raise the temperature at a rate of 10 • C/min to 180 • C, and purify the catalyst with constant temperature pre-treatment in He atmosphere for 60 min to remove impurities such as H 2 O and CO 2 . Afterward, after cooling to 50 • C, switch to a 9.84% CO/He mixture gas, and maintain blowing for 60 min to achieve adsorption saturation on the catalyst surface. Then, switch to He gas and maintain for 60 min to remove residual CO in the tube. Finally, continuously introduce He gas and heat up to 900 • C at a heating rate of 10 • C/min under the He atmosphere for testing. CO-DRIFTS was tested on the Nicolet 20, Thermo Fisher Scientific Inc., Waltham, MA, USA. Figure 1 presents the results of the performance evaluation of Ce-MnO x composite oxide catalysts with varying proportions. As can be seen, the CO conversion efficiency increases with increasing catalyst temperature, while the catalyst performance first increases Nanomaterials 2023, 13, 2158 4 of 12 and then decreases with decreasing Ce content. The catalytic efficiency of a single MnO x catalyst is relatively poor, with a CO conversion efficiency of only 46.57% at 140 • C and 95.4% at 170 • C. However, the introduction of Ce significantly enhances the catalytic performance, drastically reducing the catalytic temperature. All of the Ce-MnO x catalysts achieve CO conversion rates above 96% at 140 • C, and the Ce-MnO x catalyst with the greatest catalytic activity has a Ce:Mn ratio of 1:1. At a temperature of 100 • C, the CO conversion rate reaches 91.98%, while the conversion rate of CO of single MnO x catalyst is only 7.66%. Furthermore, the Ce-MnO x catalyst demonstrates an exceptional CO conversion rate of 99.96% at a higher temperature of 140 • C. Further elaboration on the catalytic mechanism will be discussed in detail later.

Catalyst Performance Evaluation
Inc, Massachusetts, America. Figure 1 presents the results of the performance evaluation of Ce-MnOx composite oxide catalysts with varying proportions. As can be seen, the CO conversion efficiency increases with increasing catalyst temperature, while the catalyst performance first increases and then decreases with decreasing Ce content. The catalytic efficiency of a single MnOx catalyst is relatively poor, with a CO conversion efficiency of only 46.57% at 140 °C and 95.4% at 170 °C. However, the introduction of Ce significantly enhances the catalytic performance, drastically reducing the catalytic temperature. All of the Ce-MnOx catalysts achieve CO conversion rates above 96% at 140 °C, and the Ce-MnOx catalyst with the greatest catalytic activity has a Ce:Mn ratio of 1:1. At a temperature of 100 °C, the CO conversion rate reaches 91.98%, while the conversion rate of CO of single MnOx catalyst is only 7.66%. Furthermore, the Ce-MnOx catalyst demonstrates an exceptional CO conversion rate of 99.96% at a higher temperature of 140 °C. Further elaboration on the catalytic mechanism will be discussed in detail later.

XRD and EDS Characterization Results for Catalysts
The phase analysis of Ce-MnOx composite oxide catalysts in various ratios is illustrated in Figure 2. In the Ce-MnOx catalyst phase, only CeO2 can be observed when the Ce:Mn ratio steadily falls from Ce:Mn = 3:1 to Ce:Mn = 1:1, as shown in the figure, leaving the oxide peak of the Mn element undetected. As the Ce content continues to decrease, the Mn3O4 peak gradually increases, and the CeO2 peak gradually decreases. When Ce:Mn = 1:2 and Ce:Mn = 1:3, the main phase of the Ce-MnOx catalyst is Mn3O4, and the secondary phase is CeO2. A clear peak shift phenomenon appears when the phase diagram at 18° is

XRD and EDS Characterization Results for Catalysts
The phase analysis of Ce-MnO x composite oxide catalysts in various ratios is illustrated in Figure 2. In the Ce-MnO x catalyst phase, only CeO 2 can be observed when the Ce:Mn ratio steadily falls from Ce:Mn = 3:1 to Ce:Mn = 1:1, as shown in the figure, leaving the oxide peak of the Mn element undetected. As the Ce content continues to decrease, the Mn 3 O 4 peak gradually increases, and the CeO 2 peak gradually decreases. When Ce:Mn = 1:2 and Ce:Mn = 1:3, the main phase of the Ce-MnO x catalyst is Mn 3 O 4 , and the secondary phase is CeO 2 . A clear peak shift phenomenon appears when the phase diagram at 18 • is slightly enlarged, indicating that MnO x is mainly highly dispersed and amorphous when the content of Ce is high and has entered the interior of the CeO 2 lattice [21,22]. Due to the existence of defects and unstable states, amorphous MnO x has a high surface energy that is advantageous for the catalyst reaction since it promotes gas adsorption and reaction [23,24]. As a result, when Ce:Mn = 1:1, the Ce-MnO x catalyst contained a sizable amount of amorphous MnO x that facilitated the quick adsorption and desorption of CO on the catalytic surface, hence increasing CO conversion. The average grain size of test samples was calculated based on the XRD pattern and Scherrer equation, as shown in Table 1. With an increase in Mn content, the average grain size of Mn 3 O 4 increased from 17.1 nm to 20.5 nm, indicating a rise in the crystallinity of Mn 3 O 4 in catalysis with the decrease in Ce content. The average grain size of CeO 2 shows a first decreasing and then increasing trend, and CeO 2 has the smallest average particle size, measuring only 4.3 nm, when Ce:Mn is 1:1. This is due to the interaction between amorphous Mn 3 O 4 and CeO 2 , which enables some Mn x+ ions to enter the CeO 2 lattice interior and replace Ce 4+ ions, thereby forming lattice defects, which decrease the crystallinity of the CeO 2 lattice and result in a decrease in the average grain size of CeO 2 [25]. The lower the crystallinity, the more uniformly the material is dispersed, which is beneficial to the improvement in catalyst activity [26].
slightly enlarged, indicating that MnOx is mainly highly dispersed and amorphous when the content of Ce is high and has entered the interior of the CeO2 lattice [21,22]. Due to the existence of defects and unstable states, amorphous MnOx has a high surface energy that is advantageous for the catalyst reaction since it promotes gas adsorption and reaction [23,24]. As a result, when Ce:Mn = 1:1, the Ce-MnOx catalyst contained a sizable amount of amorphous MnOx that facilitated the quick adsorption and desorption of CO on the catalytic surface, hence increasing CO conversion. The average grain size of test samples was calculated based on the XRD pattern and Scherrer equation, as shown in Table 1. With an increase in Mn content, the average grain size of Mn3O4 increased from 17.1 nm to 20.5 nm, indicating a rise in the crystallinity of Mn3O4 in catalysis with the decrease in Ce content. The average grain size of CeO2 shows a first decreasing and then increasing trend, and CeO2 has the smallest average particle size, measuring only 4.3 nm, when Ce:Mn is 1:1. This is due to the interaction between amorphous Mn3O4 and CeO2, which enables some Mn x+ ions to enter the CeO2 lattice interior and replace Ce 4+ ions, thereby forming lattice defects, which decrease the crystallinity of the CeO2 lattice and result in a decrease in the average grain size of CeO2 [25]. The lower the crystallinity, the more uniformly the material is dispersed, which is beneficial to the improvement in catalyst activity [26].   In order to further observe the element distribution in the Ce-MnO x composite oxide catalyst, the catalyst element proportion with Ce:Mn = 1:1 was characterized by EDS, and the results are displayed in Figure 3. EDS analysis of Ce and Mn was applied to the selected region, and it was found that the atomic ratio of Ce and Mn is close to 1:1, which is in line with experimental expectations, and shows that Ce and Mn elements are evenly distributed in the catalyst. Combined with XRD, the oxide peak of Mn could not be detected in the phase, and it can be inferred that MnO x failed to form a crystal structure, which is mainly amorphous. In order to further observe the element distribution in the Ce-MnOx composite oxide catalyst, the catalyst element proportion with Ce:Mn = 1:1 was characterized by EDS, and the results are displayed in Figure 3. EDS analysis of Ce and Mn was applied to the selected region, and it was found that the atomic ratio of Ce and Mn is close to 1:1, which is in line with experimental expectations, and shows that Ce and Mn elements are evenly distributed in the catalyst. Combined with XRD, the oxide peak of Mn could not be detected in the phase, and it can be inferred that MnOx failed to form a crystal structure, which is mainly amorphous.

TEM Characterization of Catalyst
TEM characterization was conducted on three Ce-MnOx catalyst samples with different cerium to manganese molar ratios (3:1, 1:1, and 1:3). The results are presented in Figure  4, which demonstrates that lattice spacing of 0.318 nm and 0.325 nm corresponded to the (111) crystal plane of CeO2 for a molar ratio of cerium to manganese of 3:1. Cerium-manganese catalysts with a molar ratio of 1:1 have lattice spacing of 0.280 nm and 0.278 nm. In the report of Minakshi et al. [27], because the mean atomic number of CeO2 was higher and the defective crystal structure was lessened, they showed stronger diffraction and Kikuchi line contrast than MnO2. However, at this ratio, the amorphous Mn x+ ion in the Ce-MnOx composite oxide catalyst replaces the Ce 4+ ion, making the CeO2 lattice distorted, resulting in the (111) lattice spacing of CeO2 becoming smaller. For a catalyst with a cerium-manganese molar ratio of 1:3, the lattice spacing is 0.274 nm and 0.433 nm. The former corresponds to the (111) crystal plane of the concave CeO2, and the latter corresponds to the (112) crystal plane of Mn3O4. The results of crystal plane analysis are consistent with those of phase analysis results provided in Section 3.2. It is observed that Mn oxide exists in an amorphous state when Ce content is high.

TEM Characterization of Catalyst
TEM characterization was conducted on three Ce-MnO x catalyst samples with different cerium to manganese molar ratios (3:1, 1:1, and 1:3). The results are presented in Figure 4, which demonstrates that lattice spacing of 0.318 nm and 0.325 nm corresponded to the (111) crystal plane of CeO 2 for a molar ratio of cerium to manganese of 3:1. Ceriummanganese catalysts with a molar ratio of 1:1 have lattice spacing of 0.280 nm and 0.278 nm. In the report of Minakshi et al. [27], because the mean atomic number of CeO 2 was higher and the defective crystal structure was lessened, they showed stronger diffraction and Kikuchi line contrast than MnO 2 . However, at this ratio, the amorphous Mn x+ ion in the Ce-MnO x composite oxide catalyst replaces the Ce 4+ ion, making the CeO 2 lattice distorted, resulting in the (111) lattice spacing of CeO 2 becoming smaller. For a catalyst with a ceriummanganese molar ratio of 1:3, the lattice spacing is 0.274 nm and 0.433 nm. The former corresponds to the (111) crystal plane of the concave CeO 2 , and the latter corresponds to the (112) crystal plane of Mn 3 O 4 . The results of crystal plane analysis are consistent with those of phase analysis results provided in Section 3.2. It is observed that Mn oxide exists in an amorphous state when Ce content is high.

Analysis of the Microscopic Surface Structure of Catalyst
The surface morphology and structure of the Ce-MnO x catalyst were analyzed via SEM, as shown in Figure 5. The microstructure of the Ce-MnO x catalyst changes with the decrease of Ce content. Particles build up to form the catalyst at a Ce:Mn ratio of 3:1, resulting in a rough surface with varying pore sizes. When the Ce content is reduced to Ce:Mn = 1:1, the catalyst surface is smooth, and it can be clearly seen that the pore size is similar and evenly distributed. Ce content eventually drops to Ce:Mn = 1:3, at which point the catalyst surface is rough and has a large number of flocculent structures but comparatively few pores and smaller pore sizes. Uniform pores are visible in the morphology of the catalyst with Ce:Mn = 1:1, which results in well-scattered active sites that facilitate gas adsorption and desorption. As a result, the catalyst at a Ce:Mn of 1:1 displays the best performance.

Analysis of the Microscopic Surface Structure of Catalyst
The surface morphology and structure of the Ce-MnOx catalyst were analyzed via SEM, as shown in Figure 5. The microstructure of the Ce-MnOx catalyst changes with the decrease of Ce content. Particles build up to form the catalyst at a Ce:Mn ratio of 3:1, resulting in a rough surface with varying pore sizes. When the Ce content is reduced to Ce:Mn = 1:1, the catalyst surface is smooth, and it can be clearly seen that the pore size is similar and evenly distributed. Ce content eventually drops to Ce:Mn = 1:3, at which point the catalyst surface is rough and has a large number of flocculent structures but comparatively few pores and smaller pore sizes. Uniform pores are visible in the morphology of the catalyst with Ce:Mn = 1:1, which results in well-scattered active sites that facilitate gas adsorption and desorption. As a result, the catalyst at a Ce:Mn of 1:1 displays the best performance.

Characterization of Catalyst H2 Temperature Programmed Reduction (H2-TPR)
The redox performance of the Ce-MnOx catalyst was evaluated by H2-TPR, and the results are shown in Figure 6. With the exception of Ce:Mn = 1:3, which exhibits three reduction peaks, only two reduction peaks are observed in other component samples for Ce-MnOx catalysts below 600 °C. The generation of highly dispersed MnOx clusters by a limited number of readily reduced manganese species results in a tiny peak at 208 °C when Ce:Mn = 1:3 [28,29]. The reduction of MnO2 and Mn2O3 to Mn3O4 results in the first reduction peak, which is located at 220 °C; the second reduction peak, which is located at 350 °C, is caused by the reduction of Mn3O4 to MnO and the reduction of oxygen on the surface of CeO2 [30]. As the content of the Ce element decreases from Ce:Mn = 3:1 to Ce:Mn = 1:1, the main reduction peaks of the catalyst all migrate to the low-temperature zone, and the intensity of the corresponding peaks increases. These findings imply that when the cerium concentration decreases, the interaction between cerium and manganese

Analysis of the Microscopic Surface Structure of Catalyst
The surface morphology and structure of the Ce-MnOx catalyst were analyzed via SEM, as shown in Figure 5. The microstructure of the Ce-MnOx catalyst changes with the decrease of Ce content. Particles build up to form the catalyst at a Ce:Mn ratio of 3:1, resulting in a rough surface with varying pore sizes. When the Ce content is reduced to Ce:Mn = 1:1, the catalyst surface is smooth, and it can be clearly seen that the pore size is similar and evenly distributed. Ce content eventually drops to Ce:Mn = 1:3, at which point the catalyst surface is rough and has a large number of flocculent structures but comparatively few pores and smaller pore sizes. Uniform pores are visible in the morphology of the catalyst with Ce:Mn = 1:1, which results in well-scattered active sites that facilitate gas adsorption and desorption. As a result, the catalyst at a Ce:Mn of 1:1 displays the best performance.

Characterization of Catalyst H2 Temperature Programmed Reduction (H2-TPR)
The redox performance of the Ce-MnOx catalyst was evaluated by H2-TPR, and the results are shown in Figure 6. With the exception of Ce:Mn = 1:3, which exhibits three reduction peaks, only two reduction peaks are observed in other component samples for Ce-MnOx catalysts below 600 °C. The generation of highly dispersed MnOx clusters by a limited number of readily reduced manganese species results in a tiny peak at 208 °C when Ce:Mn = 1:3 [28,29]. The reduction of MnO2 and Mn2O3 to Mn3O4 results in the first reduction peak, which is located at 220 °C; the second reduction peak, which is located at 350 °C, is caused by the reduction of Mn3O4 to MnO and the reduction of oxygen on the surface of CeO2 [30]. As the content of the Ce element decreases from Ce:Mn = 3:1 to Ce:Mn = 1:1, the main reduction peaks of the catalyst all migrate to the low-temperature zone, and the intensity of the corresponding peaks increases. These findings imply that when the cerium concentration decreases, the interaction between cerium and manganese

Characterization of Catalyst H 2 Temperature Programmed Reduction (H 2 -TPR)
The redox performance of the Ce-MnO x catalyst was evaluated by H 2 -TPR, and the results are shown in Figure 6. With the exception of Ce:Mn = 1:3, which exhibits three reduction peaks, only two reduction peaks are observed in other component samples for Ce-MnO x catalysts below 600 • C. The generation of highly dispersed MnO x clusters by a limited number of readily reduced manganese species results in a tiny peak at 208 • C when Ce:Mn = 1:3 [28,29]. The reduction of MnO 2 and Mn 2 O 3 to Mn 3 O 4 results in the first reduction peak, which is located at 220 • C; the second reduction peak, which is located at 350 • C, is caused by the reduction of Mn 3 O 4 to MnO and the reduction of oxygen on the surface of CeO 2 [30]. As the content of the Ce element decreases from Ce:Mn = 3:1 to Ce:Mn = 1:1, the main reduction peaks of the catalyst all migrate to the low-temperature zone, and the intensity of the corresponding peaks increases. These findings imply that when the cerium concentration decreases, the interaction between cerium and manganese oxides is strengthened, promoting the reduction behavior of cerium and manganese in the sample and improving the fluidity of oxygen species. After that, the content of Ce element continues to decrease, the main reduction peak of the catalyst moves to the hightemperature zone, and the corresponding peak strength increases, and the second reduction peak is more obvious, which also reveals that the two main components of the catalyst in Ce:Mn = 1:2 and Ce:Mn = 1:3 with higher Mn content are Mn 3 O 4 . These findings are also consistent with the XRD phase analysis results found in Section 3.2.
sample and improving the fluidity of oxygen species. After that, the content of continues to decrease, the main reduction peak of the catalyst moves to the hi ature zone, and the corresponding peak strength increases, and the second redu is more obvious, which also reveals that the two main components of the cataly = 1:2 and Ce:Mn = 1:3 with higher Mn content are Mn3O4. These findings are also with the XRD phase analysis results found in Section 3.2.  Figure 7 displays the process of CO adsorption, heating, and CO2 release face of Ce-MnOx composite oxide catalysts in varying proportions. The catal two similar analytical peaks, named α and β peaks, which means that there a ferent processes for adsorbing CO and desorbing CO2. This may be related oxygen species on the catalyst: α peak is the analytical peak of CO and active the catalyst surface to generate CO2; β peak is the analytical peak of CO and cat oxygen to generate CO2 [31]. The lowest temperature of 113.7 °C obtained as C A reduction in Ce concentration leads to a low-temperature shift and is foll high-temperature shift for the αpeak. Meanwhile, the β peak gradually moves temperature with the decrease in Ce content, and the peak area increases cont well. The results reveal that with the decrease in Ce content, the lattice oxyg gradually increases. The catalytic activity of the catalyst is generally related to [32]. The lower the α peak temperature, the more conducive to the evolution o the higher the catalytic activity. It is consistent with the optimum catalytic act MnOx composite oxide catalyst at Ce:Mn of 1:1 that the analytical peak temp CO combined with surface active oxygen to generate CO2 is the lowest at C This might be due to the abundance of amorphous MnOx exposing more act sites, increasing the quantity of active oxygen on the surface and considerably the catalytic performance of the catalyst.  Figure 7 displays the process of CO adsorption, heating, and CO 2 release on the surface of Ce-MnO x composite oxide catalysts in varying proportions. The catalysts exhibit two similar analytical peaks, named α and β peaks, which means that there are two different processes for adsorbing CO and desorbing CO 2 . This may be related to different oxygen species on the catalyst: α peak is the analytical peak of CO and active oxygen on the catalyst surface to generate CO 2 ; β peak is the analytical peak of CO and catalyst lattice oxygen to generate CO 2 [31]. The lowest temperature of 113.7 • C obtained as Ce:Mn is 1:1. A reduction in Ce concentration leads to a low-temperature shift and is followed by a high-temperature shift for the αpeak. Meanwhile, the β peak gradually moves to the high temperature with the decrease in Ce content, and the peak area increases continuously as well. The results reveal that with the decrease in Ce content, the lattice oxygen content gradually increases. The catalytic activity of the catalyst is generally related to the α peak [32]. The lower the α peak temperature, the more conducive to the evolution of CO 2 , and the higher the catalytic activity. It is consistent with the optimum catalytic activity of Ce-MnO x composite oxide catalyst at Ce:Mn of 1:1 that the analytical peak temperature for CO combined with surface active oxygen to generate CO 2 is the lowest at Ce:Mn = 1:1. This might be due to the abundance of amorphous MnO x exposing more active catalyst sites, increasing the quantity of active oxygen on the surface and considerably enhancing the catalytic performance of the catalyst.

In situ Infrared Diffuse Reflection Characterization of Catalysts
According to the results from the evaluation of catalytic activity, the Ce-MnOx composite oxide catalyst with Ce:Mn ratio of 1:1, which demonstrated the best performance, was adopted. A sample weighing 0.08 g was selected for testing. The sample was processed for 10 min at 120 °C in a nitrogen-purge environment to remove the residual water. After the treatment was completed, the sample was cooled to room temperature, and a mixed gas (N2 used as equilibrium gas, with 16% O2 concentration and CO concentration of 6000 ppm) was introduced and gradually heated up from 60 °C to 120 °C. In situ infrared spectroscopy was carried out when the temperature stabilized at each testing temperature, and the results are displayed in Figure 8. The absorption peak at 2343 cm −1 in the figure gradually increases with the increase in temperature, which is attributed to CO2 [32]. With the increment in temperature, the catalytic activity of the sample increases, and a considerable amount of CO is converted to CO2, resulting in an elevation of the peak here. The absorption peaks at 2119 cm −1 and 2181 cm −1 are CO absorption peaks, which gradually decrease with the increment in temperature [32][33][34]. The absorption peaks of 2895 cm −1 and 2922 cm −1 are attributed to the stretching vibration of the C-H bond [35]. The absorption peaks from 1200 cm −1 to 1800 cm −1 are connected to the vibration of carbonate species [35][36][37][38], and peak strength increases with the rising temperature.

In Situ Infrared Diffuse Reflection Characterization of Catalysts
According to the results from the evaluation of catalytic activity, the Ce-MnO x composite oxide catalyst with Ce:Mn ratio of 1:1, which demonstrated the best performance, was adopted. A sample weighing 0.08 g was selected for testing. The sample was processed for 10 min at 120 • C in a nitrogen-purge environment to remove the residual water. After the treatment was completed, the sample was cooled to room temperature, and a mixed gas (N 2 used as equilibrium gas, with 16% O 2 concentration and CO concentration of 6000 ppm) was introduced and gradually heated up from 60 • C to 120 • C. In situ infrared spectroscopy was carried out when the temperature stabilized at each testing temperature, and the results are displayed in Figure 8. The absorption peak at 2343 cm −1 in the figure gradually increases with the increase in temperature, which is attributed to CO 2 [32]. With the increment in temperature, the catalytic activity of the sample increases, and a considerable amount of CO is converted to CO 2 , resulting in an elevation of the peak here. The absorption peaks at 2119 cm −1 and 2181 cm −1 are CO absorption peaks, which gradually decrease with the increment in temperature [32][33][34]. The absorption peaks of 2895 cm −1 and 2922 cm −1 are attributed to the stretching vibration of the C-H bond [35]. The absorption peaks from 1200 cm −1 to 1800 cm −1 are connected to the vibration of carbonate species [35][36][37][38], and peak strength increases with the rising temperature. Due to the low temperature at which the CO reaction is catalyzed by the C catalyst, the L-H mechanism (Langmuir-the Hinshelwood mechanism) should lowed, which has a low activation energy requirement. Combined with in situ spectroscopy and the L-H mechanism, it can be inferred that CO   Due to the low temperature at which the CO reaction is catalyzed by the Ce-MnO x catalyst, the L-H mechanism (Langmuir-the Hinshelwood mechanism) should be followed, which has a low activation energy requirement. Combined with in situ infrared spectroscopy and the L-H mechanism, it can be inferred that CO and O 2 in the mixed gas are adsorbed on the catalyst surface, and the adsorbed [CO] and [O] are activated into active intermediate carbonate in the active center of the catalyst and then decomposed into CO 2 [39,40].

Conclusions
(1) The catalytic efficiency of pure-phase MnO x catalyst is 95.4% at 170 • C. While the performance of the Ce-doped catalyst is greatly improved, the Ce-MnO x catalyst can achieve more than 96% CO conversion rate at 140 • C. As the amount of Ce element diminishes, the catalyst performance initially rises and then falls. The best catalyst performance is achieved when the ratio of Ce to Mn is 1:1, where the CO-removal rate can reach 91.98% at 100 • C and 99.96% at 140 • C.